The present invention relates to steering systems for wheeled vehicles, particularly such vehicles with at least two steerable wheels under manual control of a driver and in which the steerable wheels are independently suspended from the vehicle body frame. More generally, the invention is applicable to steering systems for wheeled vehicles with at least two steerable wheels. By the present invention, the necessary forces for causing each of the steerable wheels to properly respond to the steering control provided by the driver is effectuated between the vehicle body and each (usually independently suspended) steerable wheel control arm by a mechanical actuator preferably in the form of control cables. The coordination of the left wheel and right wheel motion is attained, not with the usual mechanical link between steering control arms for the wheels, but rather by two distinct motion-position control signals provided to each of the actuators. These two signals and, hence, the motions imparted by the respective actuators have a predetermined non-linear relation which can be produced by non-linear coupling mechanism or by an electronic digital or analog computer. An advantage of the system of the invention is that the necessity for a conventional steering column is eliminated along with the safety hazard that such columns present for the driver in the event of a mishap. Another advantage is that it facilitates a very large angle of vertical travel for the independent suspension arms which is desirable for some vehicles and virtually impossible to obtain in conventional steering systems. It is particularly suitable for fore and aft extending suspension systems.
The present invention is in sharp contrast to the traditional wheeled vehicle steering system. The structure, adjustment, and alignment of the steering mechanism of a wheeled vehicle is a somewhat complicated matter involving various relationships of each steerable wheel axle to the vehicle body referred to as caster, camber, toe-in, Ackerman angle and the like. No attempt is made here to present a detailed discussion of all these factors, except as they relate to the improvements of the present invention.
For over a century, the steering system for automotive vehicles has utilized a front axle which does not rotate about a vertical axis when the vehicle is steered to the right or the left. Rather, each front wheel is provided with a stub axle which is rotatable about a vertically disposed king-pin or equivalent pivotal mounting structure. Accordingly, it is necessary to provide means by which the front wheels turn to the left or turn to the right together in a coordinated fashion. It was long ago determined that optimum steering was not achieved with strictly parallel motion of the front wheels because the outwardly turning wheel is closer to the vehicle turning center than the inwardly turning wheel and needs to rotate through a greater angle. This refinement and provision for steering control is referred to as the Ackerman angle provision.
A common and conventional wheeled vehicle steering system provides for the rotation of the steering column or steering shaft to be converted into linear motion of a link which imparts a rotating motion to the stub axle of the wheel through a steering control arm extending from the stub axle. The common means for providing coordinated motion of the two steerable wheels includes a track arm which also is fixed with respect to the stub axle on each wheel together with a track rod extending between the two track arms so that the two front wheels are constrained to move (track) in a predetermined relationship either to the right or to the left. In most cases the linkage provided does not maintain the wheels parallel but rather provides a nonlinear turning relationship which takes into account the Ackerman angle provision.
Hydraulic systems have been incorporated in conjunction with wheeled vehicle steering systems and perhaps the most common form of such hydraulic system is a power assist for the steering shaft which still requires the above-discussed track rod and track arm arrangement for coordinating turning motion of the steerable wheels. As mentioned above, the present invention eliminates reliance upon a track rod or other direct mechanical linkage between the left and right steerable wheels by causing the turning motion of each wheel to be effected by its own independent mechanical actuator.
Steering systems for wheeled vehicles have been proposed in prior patents which employ fluid-mechanical actuators in the form of hydraulic cylinders, but known steering systems employing hydraulic cylinders have commonly provided positive coordination with track rods or other direct mechanical connection between the left side and the right side wheel.
The steering system of the present invention provides adaptability to lever steering which eliminates the necessity of a steering wheel. The control of the vehicle may be accomplished with either hand or both hands. The double handle configuration of the preferred embodiment gives the operator a sense of driving a motorcycle using the handle bars except the center section of the handle bars is not present. This also allows entry and exit to the vehicle through the front of the vehicle if desired, and it removes the danger of the steering wheel and the necessary support structure upon which a person may be injured and impaled in the event of accidents. This then allows increased ride-down distance which can be incorporated into the vehicle restraint system to reduce injuries.
The Ackerman steering in the preferred embodiment is accomplished by a system of bellcranks below the seat or elsewhere in the cockpit and not with the angular relationships of a steering arm, track rod, and steering knuckle. The customary lever linkage to convert linear motion to rotational motion of the wheel about the vertical king pin axis is preferably replaced by the action of a cable winding on and unwinding from a drum pulley coaxial with the normal king pin axis. The steering actuator is effectively a rotary actuator which mounts on the steering axis of each wheel. The designer is free to set parameters of these elements to optimize suspension travel and to prevent bump steer.
Although independent control of steering of the two front wheels is mentioned in prior systems, they are not generally independent in the sense of no direct coupling between the two wheels.
U.S. Pat. No. 5,094,312 to Hakel dated Mar. 10, 1992 (U.S. Cl. 180/132) shows a vehicle steering system in which the left and right steering control arms for the front wheels are both operated by a single double-acting hydraulic piston with oppositely directed piston rods each coupled by a mechanical link to a respective steering control arm. This single piston arrangement is in effect a direct mechanical linkage from the left side wheel to the right side wheel and there are not two independent hydraulic cylinders for the respective left and right steerable wheels. U.S. Pat. No. 4,986,387 to Thompson et al., dated Jan. 2, 1991 (U.S. Cl. 180/212) shows a steerable wheeled vehicle with a steerable wheel for which turning motion is provided by hydraulic cylinder and a rack and pinion, but, since there is only one steerable wheel, it provides no teaching with respect to coordination of two steerable wheels relevant to the present invention.
U.S. Pat. No. 5,090,512 to Mullet et al., dated Feb. 25, 1992 (U.S. Cl. 180/236), like the patent to Hakel, relies on a direct mechanical connection from the left side to the right side of the steering system including a rotatable shaft and cam pulleys operating a cable arrangement for wheel steering. U.S. Pat. No. 4,006,664 to Brown has hydraulic operated steering, but like the patent to Hakel has only a single cylinder for left and right wheels, the coordinated motion of which must be provided by a linkage directly connecting left and right wheels with the single cylinder provided for their operation.
U.S. Pat. No. 2,757,014 to Schmitz, dated Jul. 31, 1956 (U.S. Cl. 280/87) discloses a steering system for a tractor having pairs of control cylinders and operating cylinders for providing steering motion to the respective front wheels which is independently controlled, there being no tie-rod connecting to the two front wheels. Although the independent control of the two front wheels is arranged so that there is a non-linear relationship between the steering motion of the left wheel and the right wheel, this non-linear relationship is not for the purpose of providing a desired Ackerman angle relationship in the steering mechanism, but, rather, is to facilitate very sharp turning of the tractor vehicle about either the left rear wheel or the right rear wheel. Accordingly, the stated objective of the steering system is solely to arrive at an extreme or limit position for the wheels in which the left wheel is turned by ninety degrees and the right wheel is turned by about fifty degrees (or vice versa). No Ackerman angle is sought or achieved for modest steering motions and, as the description states, any normal steering movement of the wheel 24 will be transmitted to the wheels 13 and 14 equally. Other features of the Schmitz disclosure make it unsuitable for an all-terrain vehicle; note that the front wheels of the vehicle are not independently suspended and there is no suggestion that it could be modified for inclusion in a vehicle with independent suspension for the steerable wheels.